Techniques for charging batteries from a supply of AC current are known in the art. Also known are battery chargers which protect the battery being charged by shutting down in the event of an over-voltage condition, and protect the battery charger circuit from the battery in the event that the battery is inadvertently connected with reversed polarity. However, if a battery is defective or inadvertently shorted out, excess charging current may flow causing damage to the battery charging circuit. Thus it is desirable for a battery charger to be provided with protection against excess current flow.
One application for a battery charging circuit is for charging an automotive type battery in a motor vehicle. Typically, the vehicle engine will drive a generator or alternator which provides a source of AC power. A battery charging circuit may be employed to take the AC power from the generator or alternator and use it to charge the battery. One problem encountered when used in this environment is due to the fact that the alternator or generator is driven by an engine which varies in speed. An increase in engine speed can result in the generated voltage increasing, and the frequency of the generator output increasing. Depending upon the design of the charging circuit, increased frequency can cause damage to certain components which must switch high power levels at the input frequency. An increased frequency is also evidence that the voltage of the input may be too high, which could result in damage to the charging circuit.
While it is possible to add any number of additional protection means to existing battery chargers, the addition of each such protection circuit requires additional complexity to the battery charger with a corresponding increase in the number of components. The addition of components increases the amount of physical space the circuit requires, and also reduces the reliability of the circuit. Thus it is desirable for a battery charging circuit to be of a simple design using relatively few components.
It is clear that there has existed a long and unfilled need in the prior art for a simple battery charging circuit which provides over-voltage protection, reverse battery polarity protection, current limiting protection, and over-frequency protection. The present invention solves these and other shortcomings of the techniques known in the prior art.